1. Field of the Invention
The present invention relates to an inverter with adjustable threshold, and more particularly, to an inverter irrelative to voltage, temperature, and process with an adjustable threshold.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a diagram illustrating a conventional inverter 100. As shown in FIG. 1, the inverter 100 comprises a N-type Metal Oxide Semiconductor (MOS) transistor Qn1, a P-type MOS transistor Qp1, an input end for receiving an input signal Vin, and an output end for outputting an output signal Vout. The source of the transistor Qp1 is coupled to a bias source VDD, the gate of the transistor Qp1 is coupled to the input end for receiving the input signal Vin, and the drain of the transistor Qp1 is coupled to the output end. The source of the transistor Qn1 is coupled to a ground end, the gate of the transistor Qn1 is coupled to the input end for receiving the input signal Vin, and the drain of the transistor Qn1 is coupled to the output end.
Please refer to FIG. 2. FIG. 2 is a diagram illustrating the input signal/output signal relation. As shown in FIG. 2, the threshold voltage is VT. When the input signal Vin is higher than the threshold VT, the output signal Vout is at a low voltage VL; when the input signal Vin is lower than the threshold VT, the output signal Vout is at a high voltage VH. For example, assume VH=5 volts, VL=0 volts, and VT=2.5 volts. Under such condition, when the input signal Vin exceeds 2.5 volts, the output signal Vout becomes 0 volts; when the input signal Vin is lower than 2.5 volts, the output signal Vout becomes 5 volts. In such manner, the input signal Vin is inverted and then output as the output signal Vout.
However, in general, the threshold is fixed. The ratio of the length/width of the PMOS and the NMOS transistors decides the threshold. Thus, if the user wants to operate the inverter with higher input voltage (such as 3˜4 volts), since the threshold remains at 2.5 volts, the inverter does not work because in this way the input voltage is always higher than the threshold and the output voltage is always kept at 0 volts (the low voltage VL). On other hand, if the user wants to operate the inverter with lower input voltage (such as 1˜2 volts), since the threshold remains at 2.5 volts, the inverter does not work because in this way the input voltage is always lower than the threshold and the output voltage is always kept at 5 volts (the high voltage VH). According to the reasons, the usage of the conventional inverter is limited.
Additionally, the threshold differs in inverters of different processes. In fact, even inverters of a same process have different thresholds: for example, PMOS transistors produced in the same process but in different batches do not necessarily output the same current under the same bias condition. Hence, the threshold of the conventional inverter is unpredictable, impacting performance.